1. Field of the Invention
The present invention relates to a wireless communication module, a communication terminal, and an impedance matching method. More specifically, the present invention relates to technology of matching impedance with an antenna in a wireless communication module incorporated into an information device (for example, a PDA).
Priority is claimed on Japanese Patent Application No. 2004-318019, filed Nov. 1, 2004, the content of which is incorporated herein by reference.
2. Description of the Related Art
In a communication terminal such as a portable mobile phone in which an antenna is equipped, in gene, circuits are designed so as to match an antenna with an RF circuit (radio amplifying circuit) in a range of the frequencies used. In wireless communication modules, a matching circuit is typically designed so as to be incorporated into an information device. Since there may be a mismatch due to the influence of objects placed around the antenna in these communication terminals and wireless communication modules, matching circuit technology having an automatic matching capability has been proposed.
According to conventional techniques, for example, in a portable wireless communication device provided with an electric power detecting device, a device for detecting a phase of a reflected signal from the antenna, and a matching circuit for adjusting a variable impedance to match between the antenna and an RF circuit, reflection phase from the antenna and input electric power into the antenna are compared with a reference value, thereby obtaining an optimal matching. The portable wireless communication device measures a reflection phase SP from the antenna and a consumption current S1 supplied to the antenna using a phase detector and an ammeter to compare them with SP0 and S10 measured in advance in a matched state, thereby attaining matching by adjusting the matching circuit repeatedly when these values deviate from the pre-measured values (refer to Japanese Unexamined Patent Application, First Publication No. 2003-174367).
Furthermore, some antenna circuits have a plurality of matching circuits, by which the transmission level is detected in front of the antenna to switch the matching circuits so that the transmission level can be maximized. These antenna circuits are designed to select a matching circuit so as to maximize the transmission level at a level-detecting portion installed closest to the matching circuits.
Furthermore, some impedance matching devices are provided with a variable impedance matching portion installed between a transmitting portion and an antenna, a standing wave ratio detecting portion, a voltage phase detecting portion, an impedance calculating portion and the like to detect the voltage standing wave ratio (VSWR) of transmission waves, thereby obtaining matching. Such impedance matching devices measure an impedance of the antenna with respect to a transmission channel and calculate a difference between the thus-measured impedance and an impedance Z0 at the transmitting portion to be matched, thereby attaining matching and also obtaining matching parameters in this instance.
In the above-described conventional technique, impedance matching requires an addition of a phase detecting circuit, a voltage standing wave ratio (VSWR) detecting circuit and the like to the circuit, resulting in problems such as increased size of the communication terminal and the like.
There is another problem in that in a wireless communication module for engaging in communications so as to be incorporated into an information device, impedance is set in advance to a predetermined value (for example, 50Ω) and the impedance deviates from the predetermined value due to electromagnetic influence from the incorporated information device, thereby making it impossible to carry out accurate impedance matching.